1. Field of the Invention
This invention generally relates to the technology of driving a motor to rotate by means of a direct PWM (pulse width modulation) system. More specifically, it relates to a technique of reducing the noise of a motor for rotating a disc by means of a direct PWM system in a disc drive apparatus for driving a disc mounted in it.
2. Description of the Related Art
Noises arise to give rise to problems in apparatus such as portable MD (Mini Disc: tradename) apparatus comprising a rotary drive system for driving a disc to rotate when the spindle motor of the rotary drive system is operated. Particularly, in the case of the PWM (pulse width modulation) system of directly supplying a pulse wide modulation signal to a spindle motor and driving the motor to rotate, noises that arise due to abrupt fluctuations of electric current at the time of a phase-switching action give rise to serious problems.
The applicant of the present patent application has proposed a technique of reducing noises by means of a soft-switching drive (control) system of reducing the electric current flowing to a spindle motor before and after a phase-switching action for the rotary drive control of the spindle motor, using such a direct PWM system (see Patent Document 1: Japanese Patent Application Laid-Open Publication No. 2002-369575).
The Patent Document 1 of the applicant of this patent application makes it possible to realize soft switching control by means of a digital circuit, which has hitherto been realized only by an analog circuit, by generating an edge signal to indicate the timing of phase-switching according to the voltage that is induced when a spindle motor is driven and controlling the highest value of the drive voltage for driving the motor according to the edge signal.
Additionally, the applicant of the present patent application disclosed a technique of appropriately controlling the revolutions of a spindle motor by selectively supplying a first timing signal indicating a timing that is computationally determined according to the phase differences of a plurality of drive signals for driving the motor and a second timing signal computationally determined according to the cycle period of one of the drive signals to the drive system in accordance with the rotary speed of the spindle motor at the time of driving the spindle motor to rotate by the direct PWM system (see Patent Document 2: Japanese Patent Application Laid-Open Publication No. 2003-9580).
However, if soft switching control is used when driving a spindle motor to rotate by means of a direct PWM system as disclosed in the Patent document 1 and, for instance, the master clock frequency of the motor control circuit of the motor is held low in order to suppress the power consumption rate while the spindle motor is driven to rotate at a high rate, there arise various problems including that the soft switch control is incomplete and the noise of the motor cannot be reduced significantly and that the PWM control is incomplete and it is not possible to supply an electric current at a rate sufficient for driving the spindle motor at high speed.
While the technique of the Patent Document 2 can be used to appropriately control the operation of driving a spindle motor to rotate by switching the mode of operation according to the rotary speed of the spindle motor (a mode for using a first timing signal and a mode for using a second timing signal), it is accompanied by a problem that it is not compatible with soft switching control for reducing the noise of the spindle motor.